1. Field of the Invention
The present invention relates to printing techniques and is particularly concerned with an orifice structure for coupling a highly-evacuated electron beam gun to a rotating print form cylinder for engraving.
2. Description of the Prior Art
The object of operating print form cylinders with the aid of an electron beam gun can be resolved in that the print form cylinder itself is supported in an evacuated chamber. Operating installations of this type of arrangement are known, for example, from U.S. Pat. No. 3,402,278; however, such installations have the disadvantage of extensive preparation and evacuation times, in particular in the presently-common great dimensions of photogravure form cylinders.
Additionally, structures are known which convey the orifice of the electron beam gun at a certain distance from the cylinder space. For thus purpose, the exterior pressure is decreased in steps to the necessary operating vacuum by means of concentric arrangements of several pressure stages.
French Pat. No. 1,480,912, German published application No. 2,207,090 and the German allowed application No. 1,515,201 all disclose systems of this type.
These structures have the decisive disadvantage that the gap between the orifice and the cylinder surface must be rather large, because the print form cylinders have relatively great concentricity errors in practical installations. Thereby, the leakage rate becomes so great that an enormous expense in regard to evacuating systems must be carried out in order to maintain the operating vacuum in the effective operational point of the electron beam. Moreover, the great leakage rate requires large suction cross sections. The frontal openings of such arrangements are therefore so large that they do not permit engraving a print form cylinder substantially to the edge, as is required in the printing technique.
Even solutions which provide a sealing foil directly over the cylinder surface, which foil or layer is permeable for the electron beam (U.S. Pat. No. 3,816,699) are not sufficient in practice, because the considerable material removal would cover the frontal surface of the foil much too rapidly, and would therefore render it impermeable for the electron beam.
Slip seals, as described in the German Gebrauchsmuster No. 75 05 278 also cannot achieve the requirements in practice. If one keeps the opening in these embodiments small (engraving to the cylinder edge), the relatively great overall height in the direction of the gun axis limits the trajectory of the removed particles such that the opening is rapidly overgrown by the removal. If one provides a large opening, it may be possible to keep the trajectory path open; however, the cylinder again can only be engraved up to an edge spacing which is somewhat larger than the opening radius. Furthermore, the structures disclosed above will have the disadvantage that the force play between vacuum and exterior pressure is hard to control. The gun is fundamentally drawn against the cylinder with the full suction force of the vacuum which, at least with sliding washers of great large opening cross sections, surely leads to difficulties at the slide point due to the high specific contact pressure. There, also, the dynamic leakage rate is very great.
In order to consider the leakage rate in sliding seals, several operating conditions must be differentiated. If the orifice operates on an unengraved cylinder surface, the leakage rate is to be low. However, this static leakage rate need not lie extremely below the dynamic leakage rate formed in that the orifice operates by half on an already engraved surface. The engraved printing elements, normally cup-shaped depressions in the cylinder surface, separated from one another, thereby continuously proceed, filled with air from the exterior pressure, beneath the orifice and are evacuated there into the vacuum of the electron gun. One can readily appreciate that not only the static, but also the dynamic, leakage rate depends to a great extent on the dimensions of the orifice opening which cannot be arbitrarily decreased in the structure suggested in the German Gebrauchsmuster No. 75 05 278, for the above-mentioned reasons.
All of the solutions described above have the disadvantage that they can only be adjusted to the many different cylinder circumferences which are conventional in practice with great difficulty. This must proceed within one operation, perhaps several times in a day.